Thyroid hormones (TH) are believed to participate in adaptation of the heart to dietary caloric restriction (CR). The effect of CR on cardiac thyroid hormone receptors α and β (TRα and TR β) has been hitherto insufficiently investigated, however, and with respect to the impact of CR on cardiac type 2 and 3 deiodinases (D2 and D3), we have not found any literature data. The aim of this study was to assess the effect of CR and age on thyroid hormone (TH) signalling (D2, D3, TR α and β) and myosin heavy chain β (BMHC) in the heart. The study was performed on 17-week- and 45-week-old male Sprague Dawley rats, fed ad libitum (AL) or restricted to 80% or 60% of AL energy intake. Plasma concentrations of TSH, total and free T3 and T4 (fT3 and fT4), protein levels of heart D2 and D3, THRα, THRβ, myosin heavy chain β (BMHC) mRNA expression and cardiac BMHC protein were determined. Morphological and endocrine parameters were influenced by age (fT3/T3, fT4/T4, THRα, BMHC protein), feeding level (TSH, T4, T3, BMHC mRNA), or by both age and feeding level (body weight, fT4, fT3). Caloric restriction reduced fT3, T4, and fT4 levels in both age groups, with additional decreases in TSH and T3 occurring in younger rats. Independently of age, cardiac BMHC expression was positively correlated with cardiac D3 and negatively with food intake and thyroid hormones. The most important finding of our study is that cardiac D3 and BMHC protein are, under conditions of differentiated dietary energy supply, directly related in both young and middle-aged rats.